High-strength extra fine metal wire

ABSTRACT

A high-strength extra fine metal wire of a diameter of 0.01-0.50 mm containing 0.60 wt %-1.20 wt % carbon, consisting of a metal structure in the form of bundle of said carbides and presenting a shape about rectangular or circular in which the ratio of the length in the longitudinal direction to the length in the direction of width in the cross section is no more than 2.5 and the mean sectional area is no more than 150×10 -4  μm 2 , and improving strength and tenacity by having a tensile strength of 300 kgf/mm 2  or over.

BACKGROUND OF THE INVENTION

The present invention relates to a high-strength extra fine metal wirewith high tenacity to be used for rubber reinforcement of tire cord,belt cord, etc., plastic reinforcement, material for electromagneticwave shield, needle material, wire saw, precision spring, wire rope,miniature rope, angling thread, etc.

Generally, extra fine metal wire is used in various modes depending onits uses: by twisting a plural number of pieces together, by weaving, inthe state of single wire, by cutting in short pieces, etc.

By the way, the properties requested of extra fine metal wire are thepossibility of finishing in an extra fine diameter, sufficiently highstrength and toughness for resisting uses in the said fields ofutilization, excellent workability in drawing and twisting, reasonablecost, etc.

However, conventional extra fine metal wires are usually manufacturedthrough several times of cold wire drawing while preventing a drop oftenacity of wire rod in each wire drawing by submitting hot rolledmaterial (high carbon steel wire rod generally) to several times ofpatenting during the processing.

For that reason, a lot of manufacturing processes were required with theprior art and the manufacturing cost was rather expensive. Moreover,patenting of extra fine metal wire is technically difficult because ofthe difficulty of temperature control, and the drawing strain was alsolimited because of wire breaking, etc. Furthermore, the true strain inthe said cold wire drawing was 2.30-3.50 or so at the maximum (truestrain ε=21n Do/Df, Do: Wire diameter before wire drawing, Df: Finishedwire diameter) and the finished extra fine metal wire usually had astrength of 300 kgf/mm² or under in tensile strength and a wire diameterof 0.15 mm or over.

A steel wire having a tempered martensite structure submitted tohardening and tempering by heat treatment is also known to the public.This steel wire is submitted to wire drawing, etc. by reducing thestrength with tempering because it is a wire rod of a comparativelylarge diameter and cannot provide a good drawability in the hardenedstate. However, it is rarely utilized in the aid fields of servicebecause it is not an extra fine metal wire of high strength.

In addition, steel wire of a diameter of 1 mm or over having a properlevel of strength and tenacity in the tempered state as oil-temperedwire is also used in a large quantity. This steel wire is prepared byoil tempering, etc. because it is poor in tenacity although it hasexcellent hardness and strength.

Namely, with the prior art, there were such problems that a hardenedsteel wire is fragile and poor in tenacity and a steel wire submitted tohardening and tempering has an improved tenacity but is difficult tocontrol on heat treatment and its strength may sharply drop depending onthe way of tempering. For that reason, wire drawing of a wire rod ofpatenting structure has so far been considered as the best way forobtaining a high-strength fine metal wire of excellent tenacity and bestdrawability from a high carbon steel wire rod.

These days, however, with the progress of technology, it has becomedifficult to sufficiently meet the required quality with an extra finewire obtained by this wire drawing with patenting, and there is now arequest for a high-strength extra fine metal wire of good productivitywhich not only is rectangular in wire diameter but also maintains a highstrength and a high tenacity and is suitable for wire drawing.

By the way, a hardened steel wire has a martensite structure and canhardly be submitted to cold working. Moreover, a steel wire of largediameter is known to improve in strength and proof stress. However, thissteel wire has poor drawability with a true strain of 0.69 or so and itstenacity is also not so high with a tensile strength of 250 kgf/mm² orso. This is probably because of an influence of its metal structure orroughing of carbide and dispersion in size, etc., according to theobservation of this inventor.

Moreover, it is patenting which has so far been considered as the bestmeans of obtaining a high-strength fine metal wire thanks to gooddrawability. It is a well known fact that this pearlite structure bypatenting is a lamellar structure of ferrite and cementite. And itsdrawability has been believed to be excellent because this cementite islamellar. Indeed, an extra fine wire of pearlite structure is submittedto wire drawing with a true strain of 3.3 or so. However, the saidcementite is about flat in shape as it appears in the micrographicstructure and its cross section is rectangular in shape. For thatreason, if you make a wire drawing of a higher drawing strain, the wirecracks with interference among its cementite layers, producing breaking,etc. (drawing limit). The drawing limit is about 3.5 in true strain atthe best. At a higher drawing strain, breaking of wire often takes placeduring the wire drawing and the tenacity also suddenly drops, making itimpossible to further improve its strength.

The present invention aims at sharply improving the drawability of awire of a certain chemical composition as well as the strength andtenacity in the state of extra fine metal wire by performing quenchingand tempering properly and by controlling the metal structure of theextra fine metal wire obtained by wire drawing at a constant level.

With the high-strength extra fine metal wire of the present invention,it has become possible to perform wire drawing with a true strain of4.0-4.7 or so and to sharply improve the strength and tenacity of thewire by eliminating interference among the carbides appearing in themicrographic structure thanks to adoption of an about rectangular orcircular shape in which the shape of the section is restricted.

Moreover, the high-strength extra fine metal wire of the presentinvention has a wide variety of applications and a high value ofutilization because it has high strength, high tenacity and excellentfatigue resistance which could never be obtained with any conventionalmetal wire, although it is made of a conventionally used carbon steelwire rod. Moreover, excellent drawability makes it possible to secure ahigh degree of processing and to also reduce the number of dies in theheat treatment process or wire drawing during the working. The effectsof this invention are really remarkable.

SUMMARY OF THE INVENTION

The inventor et al. repeated careful studies on the workability in wiredrawing as well as the strength, tenacity, etc. after wire drawing ofpearlite, martensite, sorbite, tempered martensite, etc. which are microstructure obtained by conventional patenting, quenching and quenchingand tempering. As a result, we recognized the great influence of themetal structure on the drawability, strength, tenacity, etc. of thematerial and that, especially in fine wire of carbon steel, it ispossible to obtain a high-strength extra fine metal wire better than theconventional extra fine wire by patenting by maintaining the metalstructure of the wire in a constant state with precise quenching orquenching and tempering, and finally succeeded in achieving thisinvention.

Namely, the high-strength extra fine metal wire of the present inventionis a metal wire of a diameter of 0.01-0.50 mm containing 0.60%-1.20%carbon in weight and its metal structure has the form of a bundle ofslender carbides. The wire has a shape about rectangular or circular inwhich the shape of the carbide in the cross section is 1/w≦2.5,S≦150×10⁻⁴ μm². The tensile strength of the wire is no less than 300kgf/mm².

Moreover, the high-strength extra fine metal wire of the presentinvention consists of a structure obtained by submitting a temperedmartensite structure to wire drawing.

Furthermore, the high-strength extra fine metal wire of the presentinvention has an about circular form in which no less than 90% ofcarbides have a length of 800×10⁻⁴ μm (=800 Å) or under in the directionof width in the cross section and may sometimes have a tensile strengthof 350 kgf/mm² or over.

By the way, the said carbides all have a slender shape and present anabout rectangular or circular shape in the cross section. The said shapeof carbides is, in the carbides of about rectangular shape, the shape ofcross section in a section perpendicular to the longitudinal directionof that rectangular shape and, in the above formula, 1 is the length ofthe carbide in the longitudinal direction, w is the length of thecarbide in the direction of width and S is the mean sectional area ofthe carbide.

The reason why the carbon content in the present invention was set at0.60-1.20 wt % is that this is necessary for the extra fine metal wireto have a certain fine fibrous structure after wire drawing and also tohave high strength and high tenacity. If the carbon content is lowerthan 0.60 wt %, the material cannot obtain sufficient martensite inhardening and becomes low in strength. If, on the contrary, the carboncontent is higher than 1.20 wt %, the material cannot obtain the desiredfine fibrous structure, gets poor in tenacity even if it has a highstrength and becomes unfit for wire drawing.

Moreover, the reason why the ratio of the length in the longitudinaldirection to the length in the direction of width 1/w in the shape ofcarbide in the metal structure has been set at no more than 2.5 is thatthis is necessary for obtaining the desired drawability, strength andtenacity.

Furthermore, if the mean sectional area S of the carbide is larger than150×10⁻⁴ μm², wire drawing becomes difficult and it is alsodisadvantageous from the viewpoint of strength and tenacity.

In addition, by having no less than 90% of carbides in an about circularshape of a length of 800×10⁻⁴ μm (=800 Å) or under in the direction ofwidth, it is possible to sharply improve the reduction of area in wiredrawing and to obtain a high-strength extra fine metal wire.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electron microphotograph of magnification 20,000 indicatingthe metal structure in the cross section of a high-strength extra finemetal wire which is an example of the present invention.

FIG. 2 is an electron microphotograph of magnification 20,000 indicatingthe metal structure in the cross section of a high-strength extra finemetal wire which is another example of the present invention.

FIG. 3 is an electron microphotograph of magnification 20,000 indicatingthe metal structure in the cross section of FIG. 2.

FIG. 4 is an explanatory drawing indicating the wire saw using ahigh-strength extra fine metal wire of the present invention.

EXAMPLE 1

An example of the present invention will be explained together with areference example and a conventional example. The steel wire rods usedhere are 5 different kinds of wire rod equivalent to ordinary hard steelwire rods or piano wire rods with variable carbon contents as shown inTable 1.

                  TABLE 1                                                         ______________________________________                                        Steel wire                                                                              Chemical composition (wt %)                                         rod No.   C         Si     Mn      P    S                                     ______________________________________                                        1         0.52      0.21   0.89    0.008                                                                              0.005                                 2         0.61      0.25   0.85    0.010                                                                              0.009                                 3         0.70      0.21   0.87    0.005                                                                              0.004                                 4         0.82      0.20   0.49    0.004                                                                              0.003                                 5         1.12      0.20   0.50    0.005                                                                              0.002                                 ______________________________________                                    

By using the above wire rods, we examined their metal structure, etc. bychanging the conditions of preliminary wire drawing before final wiredrawing and of final heat treatment in various ways as shown in Table 2.Table 3 indicates the results of this study.

                                      TABLE 2                                     __________________________________________________________________________           Heat Preliminary wire                                                                         Conditions of final heat treatment                     No.    treat-                                                                             drawing    Heating       Tempering                                Experi-                                                                           of ment in                                                                            Reduction                                                                           Finished                                                                           Tempera-      Tempera-                                 ment                                                                              wire                                                                             previous                                                                           of area                                                                             diameter                                                                           ture Time                                                                             Hardening                                                                           ture Time                                No. rod                                                                              process                                                                            %     mm   °C.                                                                         sec.                                                                             Liquid                                                                              °C.                                                                         sec.                                __________________________________________________________________________     1  1  Patenting                                                                          70.5  0.38 830  35 Oil   450  22                                   2  2  "    75.0  0.35 830  30 "     400  20                                   3  2  "    82.6  1.25 820  55 "     450  37                                   4  2  "    49.0  0.50 800  41 "     340  28                                   5  2  "    81.6  0.30 800  28 "     350  19                                   6  3  "    88.9  1.0  830  50 "     400  33                                   7  3  "    92.0  0.85 830  50 "     420  33                                   8  3  "    95.4  0.15 800  12 "     400   8                                   9  4  "    80.0  1.35 840  55 "     430  37                                  10  4  "    92.9  0.8  830  50 "     490  33                                  11  4  "    53.0  0.48 830  40 "     450  30                                  12  5  "    86.6  1.10 840  53 "     450  35                                  13  5  "    58.7  0.45 830  40 "     420  27                                  14  5  "    75.0  0.35 830  30 "     490  20                                  15  3  "    88.9  1.0  950  30 (Lead)                                                                              (550)                                                                              (15)                                16  4  "    75.0  1.5  950  40 (Lead)                                                                              (550)                                                                              (20)                                17  3  "    60.9  2.5  920  120                                                                              Oil   460  80                                  18  4  "    57.8  2.6  920  120                                                                              "     430  80                                  __________________________________________________________________________     *In the Table, the conditions given in () of experiments Nos. 15, 16 are      the patenting conditions.                                                

                                      TABLE 3                                     __________________________________________________________________________    After heat treatment                                                                         Shape of carbide                                               Experi-        (Shape of cross section)                                                                          Tensile                                    ment           Mean l × w                                                                     Mean                                                                              Mean section area                                                                      strength                                   No. Metal structure                                                                          Å  l/w μm.sup.2                                                                            kgf/mm.sup.2                               __________________________________________________________________________     1  Tempered martensite                                                                      2000 × 700                                                                     2.86                                                                              140 × 10.sup.-4                                                                  122                                         2  "           950 × 680                                                                     1.40                                                                              65 × 10.sup.-4                                                                   128                                         3  "          1550 × 700                                                                     2.21                                                                              109 × 10.sup.-4                                                                  128                                         4  "          1800 × 700                                                                     2.57                                                                              126 × 10.sup.-4                                                                  186                                         5  "          1050 × 700                                                                     1.50                                                                              74 × 10.sup.-4                                                                   188                                         6  "          1000 × 720                                                                     1.39                                                                              72 × 10.sup.-4                                                                   175                                         7  "          1000 × 650                                                                     1.54                                                                              65 × 10.sup.-4                                                                   165                                         8  "           950 × 700                                                                     1.36                                                                              67 × 10.sup.-4                                                                   170                                         9  "          1000 × 700                                                                     1.43                                                                              70 × 10.sup.-4                                                                   183                                        10  "          1200 × 700                                                                     1.71                                                                              84 × 10.sup.-4                                                                   158                                        11  "           800 × 680                                                                     1.18                                                                              54 × 10.sup.-4                                                                   170                                        12  "          1400 × 700                                                                     2.00                                                                              98 × 10.sup.-4                                                                   205                                        13  "          1050 × 700                                                                     1.50                                                                              74 × 10.sup.-4                                                                   235                                        14  "          1000 × 730                                                                     1.37                                                                              73 × 10.sup.-4                                                                   190                                        15  Pearlite   2500 × 700                                                                     3.57                                                                              175 × 10.sup.-4                                                                  125                                        16  "          2300 × 650                                                                     3.54                                                                              150 × 10.sup.-4                                                                  138                                        17  Tempered martensite                                                                      3500 × 870                                                                     4.02                                                                              304 × 10.sup.-4                                                                  115                                        18  "          3000 × 800                                                                     3.75                                                                              240 × 10.sup.-4                                                                  123                                        __________________________________________________________________________

After that, we performed wire drawing by selecting the degree of drawingstrain in the final wire drawing as required, and then observed andmeasured the metal texture, etc. in the cross section of the fine metalwire obtained. Table 4 indicates the results of this measurement.

                                      TABLE 4                                     __________________________________________________________________________    Final wire drawing         Shape of carbide after final wire drawing          Finished                   (Shape of cross section)                                 wire            Critical                                                                           Mean      Mean   Percentage                        Experiment                                                                          diameter                                                                           True       drawing                                                                            l × w                                                                         Mean                                                                              sectional area                                                                       of shape A                        No.   mm   strain                                                                            Drawability                                                                          strain                                                                             Å l/w μm.sup.2                                                                          %                                 __________________________________________________________________________     1    0.10 2.67                                                                              ◯                                                                        4.61 1100 × 700                                                                    1.57                                                                              77 × 10.sup.-4                                                                 78                                 2    0.05 3.89                                                                              ◯                                                                        4.61  950 × 700                                                                    1.36                                                                              67 × 10.sup.-4                                                                 92                                 3    0.20 3.67                                                                              ◯                                                                        4.51 1570 × 670                                                                    2.34                                                                              105 × 10.sup.-4                                                                67                                 4    0.15 2.41                                                                              ◯                                                                        4.61 1850 × 700                                                                    2.64                                                                              130 × 10.sup.-4                                                                35                                 5    0.03 4.61                                                                              ◯                                                                        4.71 1100 × 700                                                                    1.57                                                                              77 × 10.sup.-4                                                                 84                                 6    0.20 3.22                                                                              ◯                                                                        4.60 1000 × 700                                                                    1.43                                                                              70 ×  10.sup.-4                                                                90                                 7    0.10 4.28                                                                              ◯                                                                        4.75 1000 × 700                                                                    1.43                                                                              70 × 10.sup.-4                                                                 93                                 8    0.02 4.03                                                                              ◯                                                                        4.20  950 × 700                                                                    1.30                                                                              67 × 10.sup.-4                                                                 95                                 9    0.25 3.37                                                                              ◯                                                                        4.15 1000 × 700                                                                    1.43                                                                              70 × 10.sup.-4                                                                 92                                10    0.15 3.35                                                                              ◯                                                                        4.70 1200 × 750                                                                    1.60                                                                              90 × 10.sup.-4                                                                 70                                10    0.05 4.52                                                                              ◯                                                                        4.70  800 × 650                                                                    1.23                                                                              52 × 10.sup.-4                                                                 97                                12    0.30 2.60                                                                              ◯                                                                        4.13 1200 × 700                                                                    1.70                                                                              84 × 10.sup.-4                                                                 80                                13    0.09 3.20                                                                              ◯                                                                        4.02 1050 × 750                                                                    1.40                                                                              79 × 10.sup.-4                                                                 90                                14    0.05 3.89                                                                              ◯                                                                        4.20 1050 × 730                                                                    1.44                                                                              77 × 10.sup.-4                                                                 94                                15    0.20 3.22                                                                              ◯                                                                        3.44 2500 × 650                                                                    3.85                                                                              163 × 10.sup.-4                                                                18                                16    0.32 3.09                                                                              ◯                                                                        3.24 2300 × 650                                                                    3.54                                                                              152 × 10.sup.-4                                                                23                                17    1.75 0.71                                                                              Δ                                                                              0.80 2200 × 850                                                                    2.59                                                                              187 × 10.sup.-4                                                                32                                18    1.50 1.10                                                                              ×                                                                              0.65 2100 × 800                                                                    2.63                                                                              168 × 10.sup.-4                                                                25                                __________________________________________________________________________

(The specimens of) experiments Nos. 1-14 were all manufactured bysubmitting various fine wires of a diameter of 0.15-1.35 mm to heattreatment by changing the temperature and time of hardening andtempering. In the table, experiments Nos. 1, 2, 4 indicate referenceexamples while experiments Nos. 3, 5-14 represent examples of thepresent invention. Experiments Nos. 15-18 indicate conventionalexamples, Nos. 15 and 16 representing examples submitted to conventionalpatenting and Nos. 17, 18 representing those manufactured by performinga heat treatment to the conventional oil-tempered wires which aregenerally used as spring materials.

Tempered martensite texture in the metal structure before final wiredrawing is a structure obtained by heating a wire rod submitted to wiredrawing in the previous process at a temperature no lower than the A₁transformation point (approx. 750° C.-850° C. in this experiment) intoaustenite, changing it completely into martensite after that withquenching (oil quenching or water quenching in this experiment) and thensubmitting it to tempering at a temperature no higher than the A₁transformation point (approx. 300° C.-550° C. in this experiment).

Pearlite structure (fine pearlite structure to be more exact) isobtained by patenting which is a kind of isothermal transformationwidely adopted for this type of wire rod. It is a structure consistingof alternate lamellar sheets of ferrite obtained by heating (the wirerod) at approximately 900° C.-1,000° C. and cementite and thensubmitting it to hot bath quenching at about 550° C. by using a meltingmetal such as lead, etc. or melting salt as a cooling medium.

Critical drawing strain in final wire drawing is the drawing strainestimated as possible in manufacture judging from the drawability infinal wire drawing and is expressed with true strain ε=21 n Do/Df.

Shape of carbide indicates the shape of cross section of aboutrectangular or circular carbides in the tempered martensite structure. Atempered martensite structure has a random arrangement of carbides inwhich the structure is in a somewhat collapsed state. It was ratherdifficult to check the shape of cross section of each carbide in thisstate but we judged this shape by taking a large number of microscopicphotos continuously in the longitudinal direction.

Shape of carbide after final wire drawing is the shape of carbideappearing on the microscope in the metal structure in the cross section.In that case, the carbides differ from the carbides after heat treatmentin the way of arrangement: while the carbides after heat treatment andbefore final wire drawing as arranged at random as mentioned before,those after final wire drawing converge in one direction (direction ofwire drawing). For that reason, in the extra fine metal wire of thepresent invention, all shapes of the carbides in the metal structure inthe cross section are equal to the shape of cross section of thecarbides.

The shape of the said carbides is not uniformly rectangular but is oftencurved. In the case of curved carbides, the length of carbide wasdetermined as the length obtained by straightening the curved carbide.

As for the distinction between longitudinal direction and direction ofwidth of the carbides, the longer or wider side was named as length inlongitudinal side 1 and the shorter or narrower side as length indirection of width w. The shape of carbides was named as about circularif the 1/w ratio is about 1.5 or under and as about rectangular if thisratio is larger than above. Moreover, the shape of a carbide with alength in the direction of width of 800×10⁻⁴ μm (=800 Å) or under and a1/w ratio no more than 1.5 was indicated as shape A. Rate of occupationmeans the percentage of the shape A against the entire area (of themetal).

The photo in FIG. 1 is a microphotograph of experiment No. 10 in whichthe white grains represent carbides. This is an electron microphotographof magnification 20,000 and corroded for approximately 15 seconds with acorrosive solution (4% picric acid alcohol solution), clearly showingthe shape of the carbides. The microphotographs given in FIG. 2 and FIG.3 represent the cross section and the longitudinal section of theexperiment No. 11 respectively.

Next, we measured tensile strength, fracture elongation, reduction ofarea, fatigue strength ratio and knot strength ratio as mechanicalproperties of extra fine wire after wire drawing in the aboveexperiments. Table 5 indicates the result of those measurements.

In the table, fatigue strength ratio is a ratio of limit fatiguestrength (kgf/mm²) to tensile strength of individual wires, limitfatigue strength being defined as the stress of 10⁷ times of repetitionat 20° C. performed by using a Hunter's fatigue tester and is expressedin index against the fatigue strength ratio of the wire of experimentNo. 15.

Knot strength ratio is a ratio (%) of knot strength to tensile strength,and it is more advantageous if the value in Table 5 is larger.

No measured value is indicated for super extra fine wires (experimentsNos. 2, 5, 8, 11, 14) for which the measurement of limit fatiguestrength is particularly difficult and for fairly large wires(experiments Nos. 17, 18) for which the comparison is not suitable.

                  TABLE 5                                                         ______________________________________                                        Physical properties after final wire drawing                                                          Re-   Fatigue                                         Experi-                                                                             Tensile  Elonga-  duction                                                                             strength                                                                              Knot strength                           ment  strength tion     of area                                                                             ratio,  ratio                                   No.   kgf/mm.sup.2                                                                           %        %     Index   %                                       ______________________________________                                         1    230      3.1      54     80     58.5                                     2    280      2.8      48    --      57.6                                     3    310      2.9      52    105     59.8                                     4    285      3.2      53    100     61.3                                     5    340      2.9      54    --      59.8                                     6    345      2.8      51    110     61.0                                     7    350      2.9      52    110     60.7                                     8    360      3.0      51    --      61.0                                     9    365      2.7      48    120     60.3                                    10    310      2.9      52    115     61.4                                    11    430      3.0      45    --      58.1                                    12    345      2.8      47    110     58.6                                    13    390      2.8      51    105     59.3                                    14    410      2.7      46    --      58.0                                    15    280      2.9      41    100     53.1                                    16    290      2.8      46     97     55.0                                    17    152      2.3      35    --      47.3                                    18    175      2.1      32    --      45.1                                    ______________________________________                                    

From Tables 4 and 5, we confirmed the following:

In experiments Nos. 15, 16 which consisted in drawing wires having apearlite structure, the tensile strength was 280 kgf/mm² and 290kgf/mm², the elongation was 2.9% and 2.8%, the reduction of area was 41%and 46%, the fatigue life was 100 and 97 and the knot strength ratio was53.1% and 55.0% respectively with wire diameters of 0.20 mm and 0.32 mm.

In experiments Nos. 17, 18 which consisted in drawing wires having atempered martensite structure found in the conventional spring material,etc., the tensile strength was 152 kgf/mm² and 175 kgf./mm², theelongation was 2.3% and 2.1%, the reduction of area was 35% and 32% andthe knot strength ratio was 47.3% and 45.1% respectively with wirediameters of 1.75 mm and 1.50 mm.

On the contrary, in experiments Nos. 3, 5-14 of the present invention,the tensile strength was 310-430 kgf/mm², the elongation was 2.97-3.0%,the reduction of area was 45-54%, the fatigue life was 105-120 and theknot strength ratio was 58.0-61.4, showing a clear supremacy over theconventional examples.

Moreover, in experiments Nos. 1, 2, 4 which are reference examples ofthe same wire diameter with that of the present invention and submittedto hardening and tempering before wire drawing, the tensile strength was230-285 kgf/mm² and the fatigue life was 80-100, proving them to beinferior to the present invention.

The causes of such differences are believed to be the difference in theshape of carbides in the metal structure after wire drawing and thedegree of carbon content of the wire rods.

In the extra fine metal wire of the present invention, the excellentdrawability in the final wire drawing is also of great importance. Themain objectives of wire drawings are to obtain fine wires and to improvethe tensile strength of the wire. However, if the tensile strength getstoo large, breaking of wire takes place frequently during the wiredrawing, making wire drawing impossible. For that reason, you have toperform heat treatment (patenting, etc.) again and then wire drawing. Inthat case, if the drawability is poor, it becomes impossible to take alarge drawing strain and heat treatment must be repeated many times. Thenumber of dies also considerably increases. The patenting for this kindof wire is performed with a heating temperature of approximately 1,000°C. and a lead temperature of approximately 550° C. As the wire diametergets smaller, the temperature control becomes more difficult andbreaking of wire takes place frequently even in the lead bath process.Usually, patenting is almost impossible with a wire of a diameter of nomore than 0.6 mm.

On the contrary, with a material of excellent workability in wiredrawing as that of the present invention, it is possible to take a largedrawing strain reduce the number of times of patenting and perform wiredrawing even with fine wires of high strength, thus enabling a sharpreduction in the manufacturing cost.

Moreover, (the specimens of) experiments Nos. 7-9, 11, 13, 14 preparedby using the heat treating method which consists in first heating wirerods of a diameter of 0.1-1.6 mm at a temperature of 750° C.-805° C.,oil hardening them and then tempering them at a temperature of 300°C.-550° C. to provide them with a tensile strength of 130 kgf/mm² orover have an about circular shape of carbides including a lot ofcarbides in shape A. Their tensile strength ranged from 350 to 430kgf/mm² providing that they are high-strength extra fine wires of moreexcellent properties.

If you perform the said heat treatment by using a wire rod which wassubmitted to patenting in the intermediate heat treatment and then towire drawing, the grain size of austenite and the shape of carbides canbe made more homogenous and fine.

Moreover, the said material submitted to heat treatment can betransformed into austenite in a short time because the carbides meltwell in austenitizing if you use a material submitted to wire drawingafter patenting heat treatment, and this is effective for the refiningof carbides after heat treatment. If the diameter gets larger, theheating time required for homogenous austenitizing gets longer and thestructure in the peripheral part is liable to get coarse. In such acase, rapid heating by induction heating is effective from the viewpointof control.

EXAMPLE 2

Next, we will show an example of experiment No. 6 in which thehigh-strength extra fine metal wire of the present invention was used astire cord. Before the final wire drawing, the (material of experimentNo. 6) was finished by performing brass plating of a thickness of 0.8μon the surface. We produced a tire cord of 1×5×0.20 by twisting 5 piecesof such element wire. The mechanical properties of this tire cord wereas shown in Table 6. As compared with a conventional tire cord of1×5×0.20 for reference, this product proved to be superior in tensilestrength and fatigue resistance. Also when it is used for the beltsection for the carcass section of the tire, it is easily conceivablethat this tire cord will greatly contribute to reduction of weight, longlife and improvement of driving comfort of the tire.

                  TABLE 6                                                         ______________________________________                                                     Conventional                                                                           Steel cord according to                                              steel cord                                                                             the present invention                                   ______________________________________                                        Construction of twist                                                                        1 × 5 × 0.20                                                                 1 × 5 × 0.20                            Twisting direction                                                                           S          S                                                   Twist pitch    10.0       10.0                                                Cord diameter (mm)                                                                           0.55       0.55                                                Breaking load of                                                                             42.3       52.1                                                cord (kg)                                                                     Comparison of fatigue                                                                        100        112                                                 resistance     (standard)                                                     (3-point pulley system)                                                       ______________________________________                                    

EXAMPLE 3

We manufactured a wire saw for cutting silicon wafer (by using) thehigh-strength extra fine metal wire of the present invention inexperiment No. 11.

Before the final wire drawing of experiment No. 6 in the Table, the(material wire rod) was brass plated on the surface and submitted towire drawing in the same way (as the final wire drawing).

In this example, the work 3 is dipped in a refrigerant solution 2 mixedwith abrasive grains supplied from below as indicated in FIG. 4 andmoved at a high speed while pushing the wire 1 at the cutting positionof single crystal of silicon to be cut. 4, 5 in the figure representpulleys.

By comparing the results of this cutting with piano wire and stainlesswire, we could confirm improvements in processing speed and accuracy aswell as reduction of working loss. This is probably because thehigh-strength extra fine metal wire of the present invention wellpreserves its properties with little breaking after wire drawing thanksto its excellent workability in wire drawing, is finer and is excellentin both strength and tenacity.

Therefore, we could confirm that the high-strength extra fine metal wireof the present invention is also effective when it is used as wire sawto be adopted for cutting, grooving or grinding, etc. of precisionparts, electronic parts, various semi-conductors, diamond dies, etc.

EXAMPLE 4

So far, there are piano wire, stainless steel wire, tungsten wire, etc.as metal wires used for fishing line. Generally, fishing lines arerequested to have such basic characteristics as small water resistance,little deterioration in water such as sea water or river water,flexibility, etc. However, the fishlines made of conventional metal wirehave such problems as low knot strength ratio especially for binding afishline with another or with a fishing hook, easy breaking or poorcurling characteristic in the case of working of an impact force on thefishing line. The fishline using the high-strength extra fine metal wireof the present invention has the above-mentioned basic characteristicsand has solved the problems of the conventional products.

We bundled 7 metal wires of experiment No. 8, twisted them into astranded wire and then covered it with a synthetic resin of a thicknessof about 8 μm to manufacture a fishline. We also manufactured a similarfishline using a conventional piano wire for the sake of comparison andcompared the two products with each other. As a result, the product ofthe present invention showed a higher stranded wire strength and wasalso higher by about 10% in knot strength ratio. Moreover, it sharplydecreased the production of kinds and curls.

By the way, this invention can also be adopted for rubber reinforcementssuch as belt cord, hose wire, bead wire, etc., plastic reinforcements,shielding material for electromagnetic wave, needle material, springmaterial, wire rope, miniature rope, wire gauze, extra fine tube formedical use, woven fabric, hollow material, electric communication,cable, optical fiber cable, ski board reinforcement, glass frame,various electrode wires, etc. in addition to the said examples.

What is claimed is:
 1. A high-strength extra fine metal wire of adiameter of 0.01-0.50 mm containing 0.60 wt %-1.20 wt% carbon, having atensile strength no less than 300 kgf/mm² and being a metal structureobtained by drawing tempered martensite, said metal structure comprisinga bundle of carbides of a shape satisfying the following formula in across section thereof:

    1/w≦2.5, S≦150×10.sup.-4 μm.sup.2

where, l is the length of the carbide in the longitudinal direction, wis the length of the carbide in the direction of width S is the meansectional area of the carbide.
 2. The high-strength extra fine metalwire as defined in claim 1, wherein no less than 90% of carbides are ofan about circular shape with a length of 800×10⁻⁴ μm² or under in thedirection of width in the cross section and have a tensile strength noless than 350 kgf/mm².